A printed circuit board, or PCB, is a self-contained module of interconnected electronic components found in devices ranging from common beepers, or pagers, and radios to sophisticated radar and computer systems. The circuits are generally formed by a thin layer of conducting material deposited, or “printed,” on the surface of an insulating board known as the substrate. Individual electronic components are placed on the surface of the substrate and soldered to the interconnecting circuits. Contact fingers along one or more edges of the substrate act as connectors to other PCBs or to external electrical devices such as on-off switches. A printed circuit board may have circuits that perform a single function, such as a signal amplifier, or multiple functions.
Two other types of circuit assemblies are related to the printed circuit board. An integrated circuit, sometimes called an IC or microchip, performs similar functions to a printed circuit board except the IC contains many more circuits and components that are electrochemically “grown” in place on the surface of a very small chip of silicon. A hybrid circuit, as the name implies, looks like a printed circuit board, but contains some components that are grown onto the surface of the substrate rather than being placed on the surface and soldered.
The printing of circuits is an emerging technology that attempts to reduce the costs associated with circuit production by replacing expensive lithographic processes with simple printing operations. By printing a circuit pattern directly on a substrate rather than using the delicate and time-consuming lithography processes used in conventional circuit manufacturing, a circuit printing system can significantly reduce production costs. The printed circuit pattern can either comprise actual circuit features (i.e., elements that will be incorporated into the final circuit, such as the gates and source and drain regions of thin film transistors, signal lines, opto-electronic device components, etc.) or it can be a mask for subsequent semiconductor processing (e.g., etch, implant, etc.).
A printed wax pattern may be used as copper etch mask for creating PCBs. Laser direct imaging (LDI) is a maskless lithography method that is currently being used for this purpose. It uses a laser to write the raster image of the pattern directly on the photoresist. In order for it to be to be cost-effective, it is necessary to have special high speed resists. Also, there is no suitable method for soldermask patterning using laser direct imaging, which is the final crucial lithography step in PCB manufacturing.
Typically, circuit printing involves depositing a print solution (generally an organic material) by raster bitmap along a single axis (the “print travel axis”) across a solid substrate. Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along a print travel axis. Printing of a pattern takes place in a raster fashion, with the print head making “printing passes” across the substrate as the ejector(s) in the print head dispense individual droplets of print solution onto the substrate. At the end of each printing pass, the print head position relative to the substrate is often adjusted perpendicular to the print travel axis before beginning a new printing pass. The print head continues making printing passes across the substrate in this manner until the circuit pattern has been fully printed.
Once dispensed from the ejector(s) of the print head, print solution droplets attach themselves to the substrate through a wetting action and proceed to solidify in place. The size and profile of the deposited material is guided by competing processes of wetting and solidification. In the case of printing phase-change materials, solidification occurs when the printed spot loses its thermal energy to the substrate and reverts to a solid form. In another case, colloidal suspensions such as organic polymers and suspensions of electronic material in a solvent or carrier are printed and wet to the substrate leaving a printed feature. The thermal conditions and material properties of the print solution and substrate, along with the ambient atmospheric conditions, determine the specific rate at which the deposited print solution transforms from a liquid to a solid.
Spot placement accuracy is extremely important for semiconductor fabrication processes. Failure to have good spot placement accuracy can result in patterning defects, which can reduce the yield and lead to inconsistent device performance.
In order to have high spot placement accuracy, it is highly desirable to have print heads that have evenly spaced ejectors with the desired pitch or multiples of the desired pitch. It is therefore common to have print heads with one row of evenly spaced ejectors. This row is then rotated relative to the process motion direction such that the spacing between ejectors may be adjusted. However, some of the commercial print heads available in the market have high density of unevenly spaced ejectors. While the high density of ejectors is a desirable feature, the unevenly spaced ejectors are not. Presently, the print heads with unevenly spaced ejectors are used in certain systems to produce high spot placement accuracy by selecting ejectors that do not have significant offset from the desired location and printing at low addressability. This limits the number of ejectors that qualify for printing and therefore reduces the advantage of having high density of ejectors in the print head.
Some of the print heads have offsets in the ejector locations, which may be deliberately introduced or caused by other production limitations. For example, a print head with an ejector pitch of 169.32 μm (corresponding to 150 dpi) will give ideal printed spot positions at multiples of 150 DPI (600 DPI, 1200 DPI, etc.). However, if some of the ejectors are not in the ideal location of the 169.32 μm but have an offset, the printed spots will have a spot placement accuracy limited by the offset when printed at 150 DPI.
Thus, there is a need for a method of printing with high spot placement accuracy using print heads with random/unevenly spaced ejector locations and coarse alignment of the multiple print heads.